In a number of applications, there is need for a strong, tough, transparent window material having a high transmissivity throughout a wide range of electro-magnetic radiation. Such a window material is useful, for example, for covering a port used in connection with instrumentation for detecting electro-magnetic radiation. Materials which have been used for producing a transparent body include metal fluorides, particularly magnesium fluoride (U.S. Pat. No. 3,589,880 issued Jun 29, 1971 to Clark; U.S. Pat. No. 3,294,878 issued Dec. 27, 1966 to Carnall, Jr., et al.; U.S. Pat. No. 3,431,326 issued Mar. 4, 1969 to Letter), aluminum oxynitride (U.S. Pat. No. 4,520,116 issued May 28, 1985 to Gentilman, et al.), aluminum niobate or tantalate (U.S. Pat. No. 4,047,960 issued Sep. 13, 1977 to Reade), and solid solutions of alumina, silica and other oxides (U.S. Pat. No. 4,009,042 issued Feb. 22, 1977 to Wittler) and alumina, with minor amounts of spinel (U.S. Pat. No. 3,026,210 issued Mar. 20, 1962 to Coble).
Methods for ceramic production involving a closed porosity formation step followed by a hot isostatic pressing step have included U.S. Pat. No. 4,524,138 issued Jun. 18, 1985 to Schwertz, et al., for production of silicon carbide/boron carbide bodies and U.S. Pat. No. 4,461,750 issued Jul. 24 1984 to Chess et al., for production of ternary alkaline earth-rare earth sulfide bodies. Such methods, however, do not address the problems involved in producing a body which is transparent in the ultraviolet region and has the desired hardness and strength characteristics.
Methods have also been developed for production of transparent bodies substantially from a magnesia-alumina spinel. U.S. Pat. No. 3,974,249 issued Aug. 10, 1976 to Roy, et al., U.S. Pat. No. 3,768,990 issued Oct. 30, 1973 to Sellers, et al., U.S. Pat. No. 3,531,308 issued Sep. 29, 1970 to Bagley. Polycrystalline bodies of spinel are, in general, more easily formed than single-crystal or fusion-cast spinel or sapphire.
Previous materials and methods for production of a sintered transparent window have suffered from a number of difficulties. These materials have been deficient in transmittance in certain wavelength ranges, particularly ultraviolet ranges, for example wave lengths from about 0.2 micrometers (microns) to about 0.4 microns, as well as visible and infrared wave lengths up to about 6 microns.
Previous materials were susceptible to abrasion or erosion, for example, from high velocity impaction of dust or sand particles or rain or cloud droplets.
Previous materials were often unstable under conditions of long exposure to ultraviolet light, such that exposure to sunlight or to ultraviolet light with an intensity of about 700 microwatts/cm.sup.2, on the order of 0.25 hours or more caused a reduction of the transmittance properties of the material.
Previous materials have been difficult to form with the desired structural strength. In some applications it is desired to produce a transparent window which can withstand mechanical stress on the order of a pressure of about 15 psi (0.1 MPa), but which will preferably rupture when subjected to a pressure of about 25 psi (0.17 MPa) or more.
Certain previous materials, e.g. MgO, are hygroscopic and become cloudy upon exposure to moisture, rendering the optical qualities of the material unacceptable.
Previous production methods have been costly to practice and have required a number of difficult steps making the windows impractical to produce in quantity.
Accordingly, a number of advantages would be realized by methods and materials for producing a transparent window having high transmissivity, particularly in the ultraviolet spectral region, as well as in the infrared region (e.g., about 3 to 5 microns), high resistance to erosion or abrasion, low susceptibility to deterioration from ultraviolet light, a desired degree of strength, low cost and ease of manufacture.